This invention is directed to a coupling for tubing. More specifically, this invention is directed to a heat recoverable metal coupling for metal tubing.
A variety of heat recoverable metal couplings have been developed for joining tubing and the like together for such applications as aircraft hydraulic systems and the like. One such device is disclosed in Harrision et al, HEAT RECOVERABLE METALLIC COUPLING, Ser. No. 410,314, filed Oct. 29, 1973, which is assigned to the assignee of the present invention and is incorporated herein by reference. Other forms of such couplings include those with smooth bores, serrated bores, toothed inserts and the like. The bodies of these heat recoverable metallic coupings are often tapered at the ends to reduce the stress loading on the tubing positioned in the coupling under bending loads. By tapering the ends of the coupling, the coupling is more able to flex and distribute the load over a broader area of the tubing. This prevents or significantly reduces failure of the tubing as a result of high stress concentration.
Although high bending stress may be substantially reduced by tapering the ends of the coupling member, the flexing of tubing at the coupling causes relative movement between the ends of the coupling and the underlying tubing. This relative motion is generally longitudinal in nature and causes chafing, fretting and galling of the tubing over a period of time. This working of the tubing is especially acute in aircraft and other applications where vibration occurs for continued periods of time. The roughened, pitted area thus formed results in points of high stress concentration in the tubing which, through continued flex cycling, lead to the propagation of cracks through the tube wall eventually resulting in tube failure. Unsatisfactory attempts have been made to eliminate or reduce the amount of chafing which occurs at the ends of the coupling members. Lubricants have been applied to the interface between the coupling and tubing as an attempt to reduce the friction at the interface. It remains that certain gall prone materials are subject to the eventual failure at locations near the end of metallic couplings because of the relative motion between components.